1. Field of the Invention
The present invention relates to an exposure apparatus designed to transfer a pattern formed on a mask or reticle onto a photosensitive substrate and used in a photolithographic process for manufacturing a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like and, more particularly, to a method and apparatus for positioning a photosensitive substrate with respect to a predetermined reference plane (e.g., the imaging plane of a projection optical system).
2. Related Background Art
Conventionally, an exposure apparatus incorporates a plane position detection unit for performing proximity gap setting, focusing, leveling, and the like. Especially in a projection exposure apparatus, when a reticle pattern is to be projected/exposed on a photosensitive substrate (a wafer or glass plate on which a photoresist is coated) via a projection optical system having a high resolving power, a surface of the photosensitive substrate must be accurately aligned with the imaging plane (the projection imaging plane for the reticle pattern) of the projection optical system, that is, focusing must be performed, as disclosed in U.S. Pat. No. 4,650,983.
In order to achieve proper focusing throughout the projection field of view of the projection optical system, some consideration needs to be given to the inclination of a partial area, on the projection optical system, which enters the projection field of view, i.e., one projection/exposure area (shot area). As a technique of performing a focusing operation in consideration of the inclination of the surface of one shot area on a photosensitive substrate, a technique disclosed in U.S. Pat. No. 4,558,949 and the like is known. Especially in U.S. Pat. No. 4,383,757, there is disclosed a technique of projecting the spots of light beams on four points on a photosensitive substrate via a projection optical system, and photoelectrically detecting spot images formed by the reflected light beams, thus performing focusing and inclination correction (leveling) with respect to the photosensitive substrate.
A multi-point oblique incident type focus detection system like the one disclosed in, e.g., U.S. Pat. No. 5,118,957 is also known as a system developed from the oblique incident type focus detection system disclosed in U.S. Pat. No. 4,558,949. In this system, pin hole images are projected on a plurality of points (e.g., five points) in a shot area on a projection optical system by an oblique incident scheme without the mediacy of a projection optical system, and the respective reflected images are received by a two-dimensional position detection element (CCD) at once. The system is generally called an oblique incident type multi-point AF system, which can execute focus detection and inclination detection with high precision.
As a conventional projection exposure apparatus, a reduction projection exposure apparatus of a step-and-repeat scheme, a so-called stepper, is widely used. This apparatus is designed to sequentially move shot areas on a photosensitive substrate into the projection field of view (exposure field) of a projection optical system to position them and expose a reticle pattern image on each shot area.
FIG. 27 shows the main part of a conventional stepper. Referring to FIG. 27, a pattern image on a reticle 51 is projected/exposed on each shot area on a wafer 53, on which a photoresist is coated, via a projection optical system 52 with exposure light EL from an illumination optical system (not shown). The wafer 53 is held on a Z leveling stage 54. The Z leveling stage 54 is mounted on a wafer-side X-Y stage 55. The wafer-side X-Y stage 55 performs positioning of the wafer 53 within a plane (X-Y plane) perpendicular to an optical axis AX1 of the projection optical system 52. The Z leveling stage 54 sets the focus position (the position in a direction parallel to the optical axis AX1) of an exposure surface (e.g., an upper surface) of the wafer 53 and the inclination angle of the exposure surface in designated states.
A movable mirror 56 is fixed on the Z leveling stage 54. A laser beam from an external laser interferometer 57 is reflected by the movable mirror 56 so that the X- and Y-coordinates of the wafer-side X-Y stage 55 are constantly detected by the laser interferometer 57. These X- and Y-coordinates are supplied to a main control system 58. The main control system 58 controls the operations of the wafer-side X-Y stage 55 and the Z leveling stage 54 through a driving unit 59 so as to sequentially expose pattern images of the reticle 51 on the respective shot areas on the wafer 53 by the step-and-repeat scheme.
In this case, the pattern formation surface (reticle surface) on the reticle 51 and the exposure surface of the wafer 53 need to be conjugate to each other with respect to the projection optical system 52. However, the reticle surface does not vary much because of the high projection magnification and the large depth of focus. In general, therefore, an oblique incident type multi-point AF system is used to only detect whether the exposure surface of the wafer 53 coincides with the imaging plane of the projection optical system 52 within the range of the depth of focus (i.e., whether an in-focus state is achieved), thus controlling the focus position and inclination angle of the exposure surface of the wafer 53.
In the conventional multi-point AF system, illumination light with which the photoresist on the wafer 53 is not sensitized, unlike the exposure light EL, is guided from an illumination light source (not shown) via an optical fiber bundle 60. The illumination light emerging from the optical fiber bundle 60 is radiated on a pattern formation plate 62 via a condenser lens 61. The illumination light transmitted through the pattern formation plate 62 is projected on the exposure surface of the wafer 53 via a radiation objective lens 65. As a result, a pattern image on the pattern formation plate 62 is projected/formed on the exposure surface of the wafer 53 obliquely with respect to the optical axis AX1. The illumination light reflected by the wafer 53 is re-projected on the light-receiving surface of a light-receiving unit 69 via a focusing objective lens 66, a vibration plate 67, and an imaging lens 68. As a result, the pattern image on the pattern formation plate 62 is formed again on the light-receiving surface of the light-receiving unit 69. In this case, the main control system 58 vibrates the vibration plate 67 through a vibrating unit 70, and detection signals from a large number of light-receiving elements of the light-receiving unit 69 are supplied to a signal processing unit 71. The signal processing unit 71 supplies, to the main control system 58, a large number of focus signals obtained by performing synchronous detection of the detection signals in response to a driving signal from the vibrating unit 70.
FIG. 28B shows opening patterns formed on the pattern formation plate 62. As shown in FIG. 28B, nine slit-like opening patterns 72-1 to 72-9 are arranged on the pattern formation plate 62 in a crisscross form. Since these opening patterns 72-1 to 72-9 are radiated on the exposure surface of the wafer 53 from a direction crossing the X- and Y-axes at 45xc2x0, projection images AF1 to AF9 of the opening patterns 71-1 to 72-9 are arranged in the exposure field, of the projection optical system 52, formed on the exposure surface of the wafer 53 in the manner shown in FIG. 28A. Referring to FIG. 28A, a maximum exposure field 74 is formed to be inscribed to the circular illumination field of view of the projection optical system 52, and the projection images of the slit-like opening patterns are respectively projected on measurement points AF1 to AF9 on the central portion and the two diagonal lines in the maximum exposure field 74.
FIG. 28C shows a state of the light-receiving surface of the light-receiving unit 69. As shown in FIG. 28C, nine light-receiving elements 75-1 to 75-9 are arranged on the light-receiving surface of the light-receiving unit 69 in a crisscross form, and a light-shielding plate (not shown) having slit-like openings is arranged above the light-receiving elements 75-1 to 75-9. Images of the measurement points AF1 to AF9 in FIG. 28A are respectively formed again on the light-receiving elements 75-1 to 75-9 of the light-receiving unit 69. In this case, the illumination light reflected by the exposure surface of the wafer 53 in FIG. 27 is reflected by the vibration plate 67, which is present at the pupil position of the focusing objective lens 66 and also vibrates (rotates/vibrates) about an axis substantially perpendicular to the drawing surface of FIG. 27. For this reason, as shown in FIG. 28C, on the light-receiving unit 69, the positions of the projection images formed again on the light-receiving elements 75-1 to 75-9 vibrate in a direction RD as the widthwise direction of each slit-like opening.
In addition, since the images of the slit-like openings on the respective measurement points AF1 to AF9 are projected obliquely with respect to the optical axis of the projection optical system 52, when the focus position of the exposure surface of the wafer 53 changes, the re-formation position of the projection images on the light-receiving unit 69 changes in the direction RD. Therefore, by performing synchronous detection of the respective detection signals from the light-receiving elements 75-1 to 75-9 in response to the vibration signal from a vibration plate 67 in the signal processing unit 71, nine focus signals corresponding to the focus positions of the measurement points AF1 to AF9 can be obtained. The inclination angle and focus position of the exposure surface are obtained from these nine focus positions and are supplied to the main control system 58. The main control system 58 sets the focus position and inclination angle of the shot area on the wafer 53 to predetermined values through the driving unit 59 and the Z leveling stage 54. In this manner, in the stepper, each pattern image of the reticle 51 is exposed while the focus position and inclination angle of each shot area on the wafer 53 are aligned with the imaging plane of the projection optical system 52.
As described above, in the stepper, after each shot area on a wafer is positioned in the exposure field of the projection optical system, the focus position and inclination angle of the exposure surface of each shot area are detected by using the multi-point AF system, thus setting the entire exposure surface in the depth of focus of the projection optical system. For this reason, a long processing time is required for each shot area, resulting in a low throughput. As disclosed in U.S. Pat. No. 4,874,954, there is a method of eliminating such an inconvenience. In this method, while an X-Y stage is moved, focus positions are detected at predetermined points in a shot area which is to be exposed next on a wafer, and a Z leveling stage is finely moved to perform focusing with respect to the shot area. In the method, however, if a stepped portion is present in a shot area, it is difficult to perform accurate focusing with respect to the exposure surface (average plane) of the shot area. In addition, leveling of the shot area cannot be performed, and hence the entire surface cannot be set within the depth of focus of a projection optical system.
With a recent trend toward larger semiconductor elements, an increase in area of a pattern which can be transferred onto a wafer by one projection/exposure operation is required. Consequently, the field size of a projection optical system tends to increase. In addition, with a reduction in pattern size of a semiconductor element, a projection optical system is required to have a higher resolving power. It is, however, very difficult to realize both a broad field and a high resolving power. If, for example, an attempt is made to increase the resolving power while ensuring a field size equivalent to that in the prior art, the imaging performance (associated with distortion, curvature of field, and the like) cannot be maintained throughout the exposure field. Under the circumstances, in order to properly respond to the tendencies toward larger areas of transfer patterns and finer transfer patterns, a scan projection exposure apparatus has been reconsidered. This apparatus is designed to simultaneously scan a reticle and a wafer with respect to a projection optical system when a reticle pattern is projected/exposed on the wafer.
As a conventional scan exposure apparatus, an apparatus having a one-to-one magnification type reflecting projection optical system is known. In this apparatus, a reticle stage for holding a reticle and a wafer stage for holding a wafer are coupled to a common movable column and are scanned/exposed at the same speed. Since this one-to-one magnification type reflecting projection optical system uses no refracting element (lens), it exhibits a good chromatic aberration property throughout a wide exposure light wavelength range. The optical system simultaneously uses two or more line spectra (e.g., g- and h-rays) from a light source (mercury lamp) to increase the intensity of exposure light so as to allow a scan/exposure operation at a high speed. In the reflecting projection system, however, a point at which astigmatism values caused by both an S (sagittal) image plane and an M (meridional) image plane are made zero is limited to a position near an image height position separated from the optical axis of the reflecting projection system by a predetermined distance. For this reason, exposure light illuminating a reticle is shaped like a part of a narrow ring, a so-called arcuated slit.
As still another conventional scan exposure apparatus, an apparatus incorporating a refracting element is also known. In this apparatus, while the projecting magnification is increased or decreased by the reflecting element, both a reticle stage and a wafer stage are relatively scanned at a speed ratio corresponding to the projecting magnification. In this case, as a projection optical system, a system constituted by a combination of a reflecting element and a refracting element or a system constituted by only a refracting element is used. As an example of the reduction projection optical system constituted by a combination of a reflecting element and a refracting element, the system disclosed in U.S. Pat. No. 4,747,678 is available. U.S. Pat. No. 4,924,257 also discloses a method of performing step-and-scan exposure by using a reduction projection optical system capable of full field projection. In such a projection optical system incorporating a refracting element, exposure light illuminating a reticle has a rectangular or hexagonal shape.
In the scan exposure apparatus, similar to the stepper, exposure needs to be performed while an exposure surface of a wafer is aligned with the imaging plane of the projection optical system. For this reason, focusing and leveling may be performed by using the conventional multi-point AF system (FIG. 27) used by the stepper without any modification. In the conventional multi-point AF system, however, since measurement points are set in the exposure field of the projection optical system, focusing of a wafer may be made inaccurate owing to, e.g., the influence of a phase delay based on a signal processing time taken in the multi-point AF system. More specifically, in the scan exposure apparatus, a wafer is scanned with respect to the exposure field of the projection optical system. Even if, therefore, the wafer is finely moved along the optical axis of the projection optical system on the basis of focus positions detected at the respective measurement points in the exposure field, the wafer has already been moved by a predetermined distance at this time, and focusing cannot be always performed accurately. In order to prevent this, the moving speed of the wafer stage during a scan/exposure operation may be decreased. In this method, however, the exposure time required for each shot area is prolonged to cause a great reduction in throughput. In addition, in a leveling operation, similar to a focusing operation, leveling of the wafer is made inaccurate owing to the influence of a phase delay based on a signal processing time and the like.
It is an object of the present invention to provide an exposure method and apparatus which can align an exposure surface of a photosensitive substrate with a predetermined reference plane with high precision at high speed.
First, the present invention is suitable for a step-and-repeat projection exposure apparatus for sequentially transferring a mask pattern on each of a plurality of shot areas on a photosensitive substrate, which apparatus includes a projection optical system for projecting the mask pattern on the photosensitive substrate, and a substrate stage for holding the photosensitive substrate, two-dimensionally moving it within a plane perpendicular to the optical axis of the projection optical system, and also moving it along the optical axis.
The first apparatus of the present invention comprises position detection means for forming a pattern image having a predetermined shape on a photosensitive substrate and photoelectrically detecting light reflected by the photosensitive substrate to detect a position at each of a plurality of points on the photosensitive substrate along an optical axis of the projection optical system, thereby making an exposure surface of each shot area on the photosensitive substrate accurately coincide with an imaging plane of the projection optical system, calculation means for calculating an offset amount between the imaging plane of the projection optical system and an exposure surface of a next shot area, on which a pattern of the mask is to be transferred, along the optical axis on the basis of a detection signal output from the position detection means when each of a plurality of measurement points in the next shot area coincides with or approaches the pattern image having the predetermined shape, and control means for controlling movement of a substrate stage to reduce the calculated offset amount to substantially zero.
As described above, in the first apparatus, since a height position at each of a plurality of measurement points in an area, on a photosensitive substrate, which is to be exposed next, during movement of the substrate stage, focusing and leveling can be performed during movement of the substrate stage or immediately after the movement. This allows a great increase in throughput. In this case, even if there is a stepped portion in a shot area, no deterioration in focusing and leveling precision occurs.
Second, the present invention is suitable for a scan type projection exposure apparatus including a projection optical system for projecting a mask pattern on a photosensitive substrate, a mask stage capable of moving in a direction perpendicular to the optical axis of the projection optical system while holding a mask, and a substrate stage capable of two-dimensionally moving within a plane perpendicular to the optical axis of the projection optical system and also capable of moving along the optical axis while holding the photosensitive substrate. This apparatus is designed to transfer a mask pattern on each shot area on the photosensitive substrate by relatively scanning the mask stage and the substrate stage at a speed ratio corresponding to the magnification of the projection optical system.
The second apparatus of the present invention includes position detection means for forming a pattern image having a predetermined shape on a photosensitive substrate and photoelectrically detecting light reflected by the photosensitive substrate to detect a position at each of a plurality of points on the photosensitive substrate along an optical axis of a projection optical system, the position detection means having at least one measurement point at each of two sides of an exposure area (i.e., an area which is conjugate to an illumination area of exposure light incident on a reticle with respect to the projection optical system and corresponds to a projection area on which a reticle pattern is to be projected by the projection optical system) of the projection optical system in the relative scan direction of the mask and the photosensitive substrate, and control means for controlling movement of a substrate stage on the basis of detection signals sequentially output from the position detection means, during relative scan of the mask and the photosensitive substrate, such that partial areas, of a shot area on the photosensitive substrate, which are located inside the exposure area of the projection optical system continuously coincide with an imaging plane of said projection optical system.
As described above, in the second apparatus, a position at a predetermined point in a shot area on a photosensitive substrate along the optical axis of the projection optical system can be detected, before the shot area enters the exposure area of the projection optical system, by at least one measurement point set on each of the two sides of the exposure area. Therefore, during a scan/exposure operation, an exposure surface of the photosensitive substrate in the exposure area of the projection optical system can be accurately aligned with the imaging plane of the projection optical system.
In the first method of the present invention which is suitable for a scan exposure apparatus, after synchronous scan of a mask and a photosensitive substrate is started, a difference between a height position of a shot area, on the photosensitive substrate, which is separated from an exposure area of a projection optical system by a predetermined distance in a direction opposite to a scan direction, and a height position of an imaging plane of the projection optical system is detected. In addition, a height position set by a substrate stage on which the photosensitive substrate is placed is detected. When the shot area reaches the exposure area of the projection optical system, a height set by the substrate stage is set to a height obtained by adding the detected difference to the detected height, thereby accurately aligning the shot area with the imaging plane of the projection optical system.
In the second method of the present invention which is suitable for a scan exposure apparatus, after synchronous scan of a mask and a photosensitive substrate is started, a difference between an inclination amount of a shot area, on the photosensitive substrate, which is separated from an exposure area of a projection optical system by a predetermined distance in a direction opposite to a scan direction, and an inclination amount of an imaging plane of the projection optical system is detected. In addition, an inclination amount set by a substrate stage on which the photosensitive substrate is placed is detected. When the shot area reaches the exposure area of the projection optical system, an inclination amount set by the substrate stage is set to an inclination amount obtained by adding the detected difference to the detected inclination amount, thereby accurately aligning the shot area with the imaging plane of the projection optical system in a parallel manner.
According to the first method of the present invention, the height of a photosensitive substrate is detected by the position detection means at a place separated from the exposure area of the projection optical system by a distance determined by a phase delay based on a signal processing time taken by the position detection means and the feed speed of a substrate stage. Focusing based on the detected height of the shot area on the photosensitive substrate is performed when the shot area moves into the exposure area. The phase difference and the like caused by the position detection means and the like can be canceled by the time difference between these operations, thereby realizing accurate focusing.
According to the second method of the present invention, the inclination angle of the photosensitive substrate is detected by the inclination angle detection means at a place separated from the exposure area of the projection optical system by a distance determined by a phase difference based on a signal processing time taken by the inclination angle detection means and the feed speed of a substrate stage. Leveling based on the detected inclination angle of a shot area on the photosensitive substrate is performed when the shot area moves into the exposure area. The phase delay and the like of the inclination angle detection means and the like can be canceled by the time difference between these operations, thereby realizing accurate leveling.
The third apparatus of the present invention which is suitable for a scan exposure apparatus includes multi-point measurement means for measuring a height position of a photosensitive substrate, along an optical axis of a projection optical system, at each of a plurality of measurement points set in a direction perpendicular to a scan direction of the photosensitive substrate, and calculation means for obtaining a difference between an inclination angle of an exposure surface of the photosensitive substrate and that of an imaging surface of the projection optical system on the basis of a measurement result obtained by the multi-point measurement means. The apparatus further includes an inclination setting stage, arranged on a substrate stage, for setting an inclination angle in the scan direction (Y direction) of the photosensitive substrate and an inclination angle in a direction (X direction) perpendicular to the scan direction on the basis of the inclination angle difference obtained by the calculation means, and response speeds at which the inclination setting stage set inclination angles xcex8Y and xcex8X in the scan direction (Y direction) of the photosensitive substrate and the direction (X direction) perpendicular to the scan direction are set to be different from each other.
In this case, the multi-point measurement means may sample the height of the photosensitive substrate at each of the plurality of measurement points with reference to the position of the substrate stage when the photosensitive substrate is scanned through the substrate stage.
In addition, the multi-point measurement means may measure the height of the photosensitive substrate at each of a plurality of measurement points constituted by a plurality of points in an area (the exposure area of the projection optical system) conjugate to an illumination area of exposure light incident on the mask with respect to the projection optical system and a plurality of points in an area located in the upstream of the exposure area when the photosensitive substrate is scanned.
Furthermore, it is preferable that the multi-point measurement means changes the positions of the plurality of measurement points in the process of sequentially exposing a mask pattern on one shot area on the photosensitive substrate.
The fourth apparatus of the present invention which is suitable for a scan exposure apparatus includes height measurement means for measuring heights of a photosensitive substrate, along an optical axis of a projection optical system, at predetermined measurement points in an exposure area of a projection optical system and a measurement area constituted by an area located in the upstream of the exposure area when the photosensitive substrate is scanned, calculation means for obtaining a difference between an average height of an exposure surface of the photosensitive substrate and a height of an imaging plane of the projection optical system on the basis of maximum and minimum values of a plurality of height measurement results obtained by the height measurement means when the photosensitive substrate is scanned, and a height setting stage, arranged on a substrate stage, for setting a height of the photosensitive substrate on the basis of the height difference obtained by the calculation means.
In the third apparatus of the present invention, when a mask and a photosensitive substrate are synchronously scanned to expose a pattern image of the mask on the photosensitive substrate, the height of the photosensitive substrate is measured at a plurality of measurement points including an upstream measurement point in the scan direction by using the multi-point measurement means. By obtaining height information at the plurality of measurement points, a number of times, along the scan direction, the inclination angle of the photosensitive substrate is obtained. Thereafter, when a pattern image of the mask is to be exposed on an area whose inclination angle is obtained in this manner, the inclination angle of the area is set on the basis of the inclination angle obtained in advance. With this operation, even in the slit scan exposure scheme, the exposure surface of the photosensitive substrate is set to be parallel to the imaging plane of the projection optical system.
In the third apparatus, when such leveling is to be performed, the response speed for leveling in the scan direction is different from that for leveling in the non-scan direction. In order to explain the function and effect based on this arrangement, error factors in focusing and leveling in a scan exposure operation will be described. In a scan exposure apparatus, the following errors can be considered.
{circle around (1)} Focus Offset Error and Vibration Error
A focus offset error is a difference in focus position between an average plane of an exposure surface and the imaging plane of the projection optical system. A vibration error is an error caused by vibrations and the like in the focusing direction of the substrate stage in a scan/exposure operation. Such errors will be described in detail below with reference to a case wherein only autofocus control is performed, a case wherein batch exposure is performed as in the case of a stepper, and a case wherein exposure is performed by a scan scheme.
FIG. 21A shows a case wherein batch exposure is performed. FIG. 21B shows a case wherein exposure is performed by the scan scheme. Referring to FIG. 21A, an average plane 34 of an exposure surface 5a of a photosensitive substrate coincides with the imaging plane of a projection optical system, but focus positions at positions Ya, Yb, and Yc are different from the constant average plane 34 by xe2x88x92xcex94Z1, 0, and xcex94Z2, respectively. That is, focus offset amounts at the positions Ya and Yb are xe2x88x92xcex94Z1 and xcex94Z2, respectively.
In the case shown in FIG. 21B, a series of partial average planes 35A, 35B, 35C, . . . on the exposure surface 5a are sequentially aligned with the imaging plane of the projection optical system in the scan direction. Therefore, focus offset errors at the positions Ya, Yb, and Yc become 0 owing to an averaging effect. However, when an image is to be formed at the position Yb, the focus position moves from the average plane 35B to the average plane 35D by a distance corresponding to a height xcex94ZB. As a result, the image at the position Yb has a variation of xcex94ZB in the focusing direction. Similarly, images formed at the positions Ya and Yc respectively have variations of xcex94ZA and xcex94ZB in the focusing direction.
That is, in the scan scheme, although a focus offset error becomes almost 0 with respect to an uneven portion on the photosensitive substrate surface of a predetermined frequency or less, new errors (vibration errors) are caused by rolling or pitching of a substrate stage, vibrations in the focusing direction (Z-axis direction), error components caused when an autofocus mechanism and an auto-leveling mechanism follow up low-frequency air fluctuation errors, short-term wavelength variations of exposure light (KrF excimer laser or the like), and the like.
{circle around (2)} Focus Follow-up Errors, Air Fluctuation Errors, and Stage Vibration Errors
These errors are typical examples of the vibration errors mentioned in {circle around (1)}, which errors are dependent on the response frequencies of the autofocus mechanism and the auto-leveling mechanism and can be classified into the following errors:
(1) a high-frequency stage vibration error which cannot be controlled by a control system, a short-term wavelength variation error of exposure light (KrF excimer laser or the like), and the like;
(2) of air fluctuation errors, a low-frequency air fluctuation error and the like that the substrate stages follows up; and
(3) a measurement error and the like which are not considered as focus errors because the substrate stage does not follow up them, although they are included in measurement results obtained by a focus position detection system or an inclination angle detection system.
{circle around (3)} Errors Caused by Uneven Portion on Exposure Surface of Photosensitive Substrate
These errors are caused because the exposure field of the projection optical system is a two-dimensional unit plane, and measurement of focus positions with respect to an exposure surface of a photosensitive substrate is performed at an finite number of measurement points in a scan/exposure operation. The errors can be classified into two types of errors as follows:
(1) an offset error between a surface 36A to be positioned (focus plane) and an ideal focus plane, which error is based on a method of calculating the positions of measurement points when the focus plane 36A and a focus plane 36B are obtained by measuring focus positions at multiple points on the exposure surface 5a of the photosensitive substrate, as shown in FIGS. 22A and 22B; and
(2) an error caused by the difference between the scan speed and the follow-up speeds of the autofocus mechanism and the auto-leveling mechanism, the response speed of the focus position detection system, and the like.
In this case, the response speed (focus response) at which a focus position is aligned with the imaging plane of the projection optical system is determined by the time delay error shown in FIG. 22C and the servo gain shown in FIG. 22D. Referring to FIG. 22C, a curve 37A represents a focusing-direction driving signal (target focus position signal) for aligning a series of partial areas of the exposure surface 5a of the photosensitive substrate with the imaging plane of the projection optical system, and a curve 38A represents a signal (follow-up focus position signal) obtained by converting the moving amounts of the series of partial areas of the exposure surface 5a in the focusing direction into a driving signal. The curve 38A is delayed with respect to the curve 37A by a predetermined period of time. Similarly, referring to FIG. 22D, a curve 37B represents a target focus position signal for the series of partial areas of the exposure surface 5a of the photosensitive substrate, and a curve 38B represents a follow-up focus position signal for the series of partial areas of the exposure surface 5a. The amplitude (servo gain) of the curve 38B is smaller than that of the curve 37B by a predetermined amount.
In the third apparatus of the present invention, in order to remove these errors, the response characteristic of the leveling mechanism in the scan direction is set to be different from that in the non-scan direction. As a multi-point measurement system for the auto-leveling mechanism in the present invention, an oblique incident type multi-point focus detection system is assumed. It is an object of the present invention not to consider an average plane of an exposure surface of a photosensitive substrate in a predetermined area in the exposure field of the projection optical system but to minimize the maximum value of offsets between the respective points on an exposure surface and the imaging plane of the projection optical system in the predetermined area. When the maximum value of offsets between almost all the points on an exposure surface of the photosensitive substrate and the imaging plane of the projection optical system is minimized in a predetermined area in the exposure field of the projection optical system, this exposure field is called a xe2x80x9cgood fieldxe2x80x9d.
Assume that there are a large number of focus position measurement points (not shown) in a slit-like exposure area 24 conjugate to an illumination area of exposure light incident on a mask with respect to the projection optical system, as shown in FIG. 23.
Referring to FIG. 23, assuming that one shot area SAij on a photosensitive substrate is scanned with respect to the exposure area 24 in the Y direction at a speed V/xcex2, the width of the shot area SAij in the scan direction is represented by WY; the width in the non-scan direction, WX; and the width of the exposure area 24 in the scan direction, D. Focus positions at a large number of measurement points in a central area 24a in the exposure area 24 are averaged to obtain a focus position of an average plane at the central point of the exposure area 24. In addition, an inclination angle xcex8Y of the average plane in the scan direction is obtained by, e.g., least square approximation on the basis of focus positions at the measurement points in measurement areas 24b and 24c on two sides of the exposure area 24 in the scan direction. Furthermore, an inclination angle xcex8X of the average plane in the non-scan direction is obtained by, e.g., least square approximation on the basis of focus positions at the measurement points in the measurement areas 24b and 24c on two sides of the exposure area 24 in the non-scan direction. Letting fm [Hz] be the response frequency of leveling in the scan direction, and fm [Hz] be the response frequency of leveling in the non-scan direction, the values of fm and fn are independently set.
The period of periodic curving of the shot area SAij on the photosensitive substrate in the scan direction is represented by a curving parameter F as a ratio with respect to the width WY in the scan direction (a similar curving period is set in the non-scan direction). A focus error at each measurement point in the exposure area 24 with such periodic curving is represented by the sum of the absolute value of the average of focus errors in a scan operation and ⅓ the amplitude of the amplitude of each focus error in the scan operation. In addition, the amplitude of the periodic curving of the curving parameter F is normalized to 1, and an error parameter S exhibiting the maximum value of the focus errors at the respective measurement points when the curving parameter is represented by F is represented by a ratio with respect to the curving parameter F. That is, the following equations can be established:
F=period of curving/WYxe2x80x83xe2x80x83(1)
S=maximum value of focus errors/Fxe2x80x83xe2x80x83(2)
FIG. 24A shows the error parameter S with respect to the curving parameter F in a case wherein the response frequency fm of leveling in the scan direction is equal to the response frequency fn of leveling in the non-scan direction, and both the frequencies are high. Referring to FIG. 24A, a curve A1 represents the error parameter in the non-scan direction; a curve B1, the absolute value of the average of ordinary focus errors in the error parameter S in the non-scan direction; a curve A2, the error parameter S in the scan direction; and a curve B2, the absolute value of the average of ordinary focus errors in the error parameter S in the scan direction. The curves A1 and A2 represent more realistic focus errors. When the value of the curving parameter F is small, and the period of uneven portions on an exposure surface is short, the follow-up property of leveling control in the scan direction is poor (curve A2). As the period of uneven portions increases, leveling control in the scan direction follows up curving. Since no sequential change in focus position occurs in the non-scan direction unlike the scan direction, even if the curving period increases in the non-scan direction, the follow-up property (curve A1) is poorer than that in the scan direction. As described above, a focus error is preferably set such that the parameter S becomes 0.5 or less. However, overall focus errors in both the scan direction and the non-scan direction are large.
FIG. 24B shows the error parameter S with respect to the parameter F in a case wherein the response frequency fm of leveling in the scan direction is set to be higher than the response frequency fn of leveling in the non-scan direction, and both the response frequencies fm and fn are low. Referring to FIG. 24B, a curve A3 represents the error parameter S in the non-scan direction; a curve B3, the absolute value of the average of ordinary focus errors in the non-scan direction; a curve A4, the error parameter S in the scan direction; and a curve B4, the absolute value of the average of ordinary focus errors in the scan direction. As is apparent from the comparison between FIGS. 24A and 24B, the error parameter S is closer to 0.5 and the focus error is smaller in the case of low response frequencies (FIG. 24B) than in the case of almost perfect response (FIG. 24A). This is because when the auto-leveling mechanism follows up fine uneven portions on the photosensitive substrate, a deterioration in precision occurs in the slit-like exposure area 24. Note that if the response frequencies are set to be too low, the leveling mechanism cannot follow up even low-frequency uneven portions. Therefore, the response frequencies must be set to be proper values.
In the case shown in FIG. 24B, the response frequency fm of leveling in the scan direction is set to be higher than the response frequency fn of leveling in the non-scan direction for the following reason. The period of uneven portions with the parameter F becomes substantially shorter in the scan direction than in the non-scan direction in accordance with the slit width. Therefore, in order to proper follow up uneven portions on an exposure surface, the response frequency in the scan direction needs to be higher than that in the non-scan direction.
When the multi-point measurement means for the auto-leveling mechanism is to measure the height of a photosensitive substrate at a plurality of measurement points constituted by a plurality of points in an exposure area (24) of the projection optical system and a plurality of points in an area located in the upstream of the exposure area when the photosensitive substrate is scanned, focus positions at measurement points in the area in the upstream of the exposure area are pre-read. This operation is called a xe2x80x9csplit pre-readxe2x80x9d operation. In this method, the length (approach distance) by which focus positions are read by the multi-point measurement means before exposure is reduced, as compared with the method of pre-reading all the measurement points (a complete pre-read operation).
When the multi-point measurement means sequentially changes the positions of a plurality of measurement points in the process of exposing a mask pattern on one shot area on a photosensitive substrate, for example, a split pre-read operation is performed at an end portion of the shot area, and a complete pre-read operation is performed at a central portion and the subsequent portion of the shot area, while an exposure position detecting section checks the results by open loop control. With this operation, while the leveling precision is kept high, the approach distance at the end portion of each shot area can be reduced to increase the throughput.
Next, consider autofocus control in the fourth apparatus of the present invention. According to the concept of the above-mentioned good field, as shown in FIG. 23, if the focus positions at the respective measurement points in the central portion 24a of the exposure area 24 are averaged, and the plane represented by the average of the focus positions is aligned with the imaging plane of the projection optical system, a deterioration in precision may occur. FIG. 25A shows a plane 34A corresponding to the average of the focus positions at the respective measurement points on an exposure surface 5a, of a photosensitive substrate, which has an uneven portion having a height H. A difference xcex94Z3 between the plane 34A and the uneven portion in the focusing direction is larger than H/2.
In contrast to this, in the fourth apparatus of the present invention, the maximum and minimum values of the focus positions at the respective measurement points in a predetermined measurement area on the exposure surface 5a are obtained, and a plane corresponding to the intermediate focus position between the maximum and minimum values is aligned with the imaging plane of the projection optical system.
FIG. 25B shows a plane 34B corresponding to the intermediate focus position between a maximum value Zmax and a minimum value Zmin of the focus positions at the respective measurement points on the exposure surface 5a, of the photosensitive substrate, which has an uneven portion having a height H. A focus position Z34B of the plane 34B can be expressed as follows:
Z34B=(Zmax+Zmin)/2xe2x80x83xe2x80x83(3)
Subsequently, the plane 34B is aligned with the imaging plane of the projection optical system. Both a difference xcex94Z4 between the plane 34B and the exposure surface 5a in the focusing direction, and a difference xcex94Z5 between the plane 34B and the uneven portion in the focusing direction are almost H/2. That is, the maximum value of focus position errors at the respective points on the exposure surface 5a is smaller on the plane 34B in FIG. 25B than that on the plane 34A in FIG. 25A. According to the concept of the good field, therefore, an exposure surface of a photosensitive substrate can be more accurately aligned with the imaging plane of the projection optical system by the present invention.
FIGS. 26A and 26B respectively show the characteristics of the error parameters S with respect to the curving parameters F in cases wherein the response frequency fm of leveling in the scan direction is set to be equal to the response frequency fn in leveling in the non-scan direction, and the two frequencies are set to be high, as in the case shown in FIG. 24A, while autofocus control based on the averaging process shown in FIG. 25A, and autofocus control based on the average of the maximum and minimum values shown in FIG. 25B are respectively performed. Referring to FIG. 26A showing the case based on the averaging process, curves A5 and B5 respectively represent the error parameters S in the non-scan direction; and curves A6 and B6, the error parameters S in the scan direction. Referring to FIG. 26B showing the case based on the average of maximum and minimum values, curves A7 and B7 respectively represent the error parameters S in the non-scan direction; and curves A8 and B8, the error parameters S in the scan direction.
As is apparent from FIG. 26B, when autofocus control is performed on the basis of the average value of maximum and minimum values, the value of the error parameter S is close to 0.5 with respect to all the curving parameters F, i.e., all the frequency bands, and the maximum value of focus errors is smaller than that in the case wherein autofocus control is performed on the basis of an averaging process.
Referring to FIGS. 22A and 22B again, consider a case wherein autofocus control is performed on the basis of the average of the maximum and minimum values of focus positions obtained at the respective measurement points in a predetermined measurement area. As shown in FIG. 22A, a plane 36A defined by a focus position difference xcex94Za with respect to the maximum value of focus positions of an exposure surface 5a having a curve with an amplitude 2xc2x7xcex94Za is aligned with the imaging plane of the projection optical system. Assume that autofocus control is simply performed with respect to the exposure surface having the curve with the amplitude 2xc2x7xcex94Za on the basis of the average of the focus positions obtained at the respective measurement points, and auto-leveling control is performed on the basis of least square approximation of the obtained focus positions. In this case, as shown in FIG. 22B, a plane 36B defined by a focus position error xcex94Zb ( greater than xcex94Za) with respect to the maximum value within the range of an amplitude xcex94Zc ( greater than 2xc2x7xcex94Za) is aligned with the imaging plane of the projection optical system in some case. Therefore, a focus error in autofocus control based on the average of the maximum and minimum values of obtained focus positions is smaller than that in autofocus control based on an averaging process, regardless of whether the auto-leveling mechanism is used or not.
In the present invention, control is performed such that a plane defined by (the maximum value Zmax of focus positions+the minimum value Zmin of the focus positions)/2 is aligned with the imaging plane. However, the depth of focus of either a projection or a recess of an exposure surface 5a of a photosensitive substrate may be required depending on a device process. Therefore, control is preferably performed such that a plane at a focus position ZMN defined by a proportional distribution represented by the following equation using predetermined coefficients M and N is aligned with the imaging plane:
ZMN=(Mxc2x7Zmax+Nxc2x7Zmin)/(M+N)xe2x80x83xe2x80x83(4)